audioflinger: flush HAL when transitioning to next direct track

Send flush command to the audio HAL when transtioning to
next track on direct output thread, even if both tracks are in the
same audio session.

Commit 43b4dcc6 to fix issue 21145353 did only flush the HAL if the
audio session was different for the new track because the logic was
copied from the offload thread.

Bug: 22019044.
Change-Id: I89b217580023ed7449a58e9bf3dc068ce7a84487